1. Field of the Invention
The present invention relates to a brake system with a brake booster, and more particularly to a counter force mechanism of a brake booster.
2. Discussion of the Prior Art
Generally, the brake system in use with a motor vehicle is made up of a brake booster with an input shaft interlocked with a brake pedal, a master cylinder interlocked with the output shaft of the brake booster, and a wheel cylinder operating by the hydraulic output of the master cylinder.
The brake booster comes in two varieties; a pneumatic pressure booster and a hydraulic pressure booster. The pneumatic pressure booster is composed of a valve body slidably disposed within a shell, a power piston provided in the valve body, a fixed pressure chamber and a variable pressure chamber disposed on both sides of the power piston, a valve mechanism for selectively connecting fluid passages located in the valve body, a valve plunger slidable disposed within the valve body and forming in part the valve mechanism, an input shaft for moving forward and backward the valve plunger to cause the valve mechanism to operate for passage switching, and an output shaft being moved forward when the power piston is moved froward.
The pneumatic pressure booster further includes a counter force mechanism. The counter force mechanism has a reaction disc, made of rubber, disposed between the output shaft and the valve plunger. When the booster is operated, the valve body and the valve plunger are simultaneously brought into contact with the reaction disc. A part of brake counter force applied to the output shaft is transmitted to the valve body, and the remaining one is transmitted to the valve plunger. The brake counter force acting on the valve plunger is transmitted to the driver via the input shaft and the brake pedal.
A servo ratio of the brake booster may be varied by varying a ratio of the counter force received by the valve body and that by the valve plunger, viz., a ratio of the pressure receiving areas of the valve body and the valve plunger.
The hydraulic pressure booster is composed of a power piston slidably disposed within a housing, a power chamber formed at one end of the power piston within the housing, a valve mechanism, provided in the power piston, for selecting connecting fluid passages in linking with the input shaft, a fluid supplying passage for communicating the power chamber with a hydraulic pressure source through the action of the valve mechanism, a discharge passage for communicating the power chamber with a reservoir through the action of the valve mechanism, and an output shaft being moved forward when the power piston is moved froward.
In the hydraulic pressure booster, a hydraulic pressure is transmitted from the power chamber to the output shaft, and then transmitted as a brake counter force to the driver.
In the conventional brake booster, the servo ratio is set at a high value so that a large brake hydraulic pressure is produced by a small pedaling force to the brake pedal. In a rapid operation for brake, the brake booster fails to follow the quick braking operation, and cannot produce a large output as expected. This fact implies that the rapid braking operation is difficult for the infirm driver, e.g., old man and woman.
In the case of the pneumatic pressure booster, upon depression of the brake pedal, the valve mechanism is switched through the action of the input shaft; a pressurized fluid is introduced into the variable pressure chamber; and the power piston and the valve body are moved forward. With the advancement of the valve body, the output shaft is advanced through the action of the reaction disc, the advancement of the output shaft generates a hydraulic pressure; a reaction or counter force to the hydraulic pressure is applied to the output shaft; and the brake counter force applied to the output shaft is distributed to the valve body and the valve plunger.
In the rapid operation for brake, before the power piston and the valve body are advanced by the pressurized fluid introduced into the variable pressure chamber, the valve plunger that is interlocked with the brake pedal through the input shaft is advanced. Most of the brake counter force applied to the output shaft is transmitted to the valve plunger. The result is that the brake counter force transmitted to the driver is considerably large.
The driver must depress the brake pedal overcoming such a large brake counter force. A pedaling force required for the rapid braking is much larger than that in a normal braking operation in which the brake pedal is gradually depressed to produce a large braking force.
This is true for the hydraulic pressure booster.